This application relates generally to microelectromechanical systems and more specifically to the reduction of leaching during the fabrication of microelectromechanical systems.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. The micromachining fabrication processes that are used in MEMS fabrication may be categorized generically into three classes: deposition, patterning, and etching. The fabrication of any specific MEMS structure will generally use some combination of these three basic techniques in some order.
Deposition refers generally to a class of techniques in which a layer of material is formed on the surface of a structural film or other material layer. Techniques used for such deposition include epitaxy, oxidation, sputtering, evaporation, various forms of chemical-vapor deposition, spin-on methods, sol-gel methods, anodic bonding, electroplating, and fusion bonding. Such deposition techniques generally produce an approximately uniform layer over the entire underlying surface. Both structural and sacrificial layers may be deposited, “structural layers” referring to those intended to form part of the final MEMS device and “sacrificial layers” referring to those meant to be dissolved later on in the process of forming the final MEMS device.
Patterning refers generally to a class of procedures that are used in preparation for removing specific portions of a uniformly deposited layer or of a structural film. Typically, a layer of photosensitive polymer (“photoresist”) is deposited using some deposition technique. The photoresist is patterned by optical exposure through a mask to print an image of the mask onto the photoresist, with the exposed photoresist being subsequently dissolved by immersion in an aqueous developer solution. Optical exposure through the mask may take place in at least three modes: contact, in which the mask touches the photoresist; proximity, in which the mask is merely brought close to the photoresist; or projection, in which an optical arrangement is used to project an image of the mask onto the photoresist.
The remaining photoresist after patterning protects the microstructure from exposure during etching, in which structural film material or portions of deposited material (as defined by the patterned photoresist) are selectively removed. Such removal may proceed by a variety of methods, including wet isotropic etching, wet anisotropic etching, plasma etching, reactive-ion etching (RIE), and deep reactive-ion etching (DRIE).
The removal of sacrificial layers is referred to as a “release,” and typically involves a chemical reaction. In surface micromachining, the sacrificial layer is generally silicon dioxide and the release involves the use of hydrofluoric acid (HF). Long exposure to HF can result in damage to the structural layers. For example, it has been reported that HF may attack the grain boundaries of polycrystalline silicon (“polysilicon”), making this structural material mechanically weaker. In addition, exposure to HF may cause dopants, which may be important for the electronic properties of the microstructure, to leach from the structural layers. In order to avoid such leaching, it is common to seek micromachining methods that limit the exposure of HF to the structural layers. Such techniques may require additional fabrication steps to minimize the extent of the exposure to structural layers or may require limiting the time of exposure to the microstructure. This may increase the complexity of the fabrication process and/or limit the exposure time to sacrificial layers to less than would otherwise be desirable.